Make AGENTS.md the canonical agent guide; CLAUDE.md only imports it #62

Agents other than Claude Code (Codex, Copilot, Cursor) read AGENTS.md, not CLAUDE.md. Claude Code only reads AGENTS.md when there is no CLAUDE.md, so the documented approach is to make AGENTS.md the real file and leave CLAUDE.md as the single line @AGENTS.md. CodoMetis.TypeKit already works this way.

• Move CLAUDE.md to AGENTS.md, keeping its history.
• Point every reference that means "the agent guide" at AGENTS.md: CONTRIBUTING, docs, test and source comments. The CHANGELOG and seed patch headers record history, so they stay as they are.

PR #51 (#60) and PR #54 (#37) both edit CLAUDE.md, so this lands after both. It is the follow-up #60 names.